In many business environments, a server database ("central") is used to store data that is pertinent to many employees or remote users of a business. The server database is typically accessible by remote computer systems ("clients") to increase the availability of information to the remote users. By providing a server database, which may be accessed by remote computer systems, dissemination of information through the company is increased. Remote access to a server database is more critical in environments where a sales force or many employees operate away from the office. As an example, the remote employees rely on the information contained within the database to be informed about inventory changes, pricing data, and company events. Rather than remain connected to the server database indefinitely and collect telecommunication charges or tie up phone lines, the remote users only intermittently connect their computers to a server for access to the server database. In these environments, the remote computer systems typically store a portion of the server database locally to support the remote application even when the client is not connected to the server. The intermittent connection is then used to send only changes made by the client application to the server and a pertinent set of changes from the server to the client. This type of remote computer system environment is called an Intermittently Connected Database (ICDB) environment. ICDBs have a wide variety of applications in sales force automation, insurance claim processing, and mobile work forces in general.
An important communication issue for this type of computer environment is the timely and efficient exchange of information between the clients and the server database. The term "database synchronization" is often used to describe the process of maintaining data consistency and integrity among server databases and client databases. There are many synchronization schemes for maintaining consistency. In some known database synchronization schemes, a time stamp is associated with the exchanged data that specifies the date of the last update exchange. The server database and the client database use the time-stamp to determine which records have been modified. Other schemes use bit-maps to mark records that have changed. Still other schemes use "before values" to track changes. Typically, only those records modified since the last data exchange are sent.
In ICDB environments, continuous access to an application requires that each client maintain a local copy of the data it shares. In addition, with intermittent connectivity, direct exchange of information between clients is not possible; consequently, some intermediary is required to facilitate inter-client communication and data sharing. Traditionally, a database server whose schema and data consists of the union of the client schemes and data fills the role of this intermediary. This server is continuously available for connection to the intermittently connected clients. Updates from a client are sent to the server. The server determines the set of clients who share in this data and need to receive a copy of the update. Thus, in this "client-centric" approach, the server has to generate a separate data set for each client. Unfortunately, this approach increases in complexity with each additional client and is limited in scalability with respect to the maximum number of clients that can be supported by a server because for every additional client, the server must repeat the same type of processing it did for other clients.
Many other techniques exist for ensuring reliable synchronization. A technique for ensuring the reliability of data synchronized for remote databases is discussed in U.S. Pat. No. 5,649,195 to Scott, et al. (the "'195 Patent"). In this system, each remote computer contains a replica of a master database. The central computer of the '195 Patent stores record changes made to the master database for a pre-defined unit of time in a file. Each change record for the same unit of time is associated with the same broadcast number and sequence number. The sequence number identifies the sequential order of the change records in the file. The last change record in the sequential order is identified as the last record for a particular broadcast number. For the next unit of time, the broadcast number is incremented and the sequence number reset. After a pre-defined unit of time has expired, the file of change records is transmitted to the remote replica databases. The remote computers use the broadcast and sequence numbers to verify the order of the change records to determine whether all change records for a broadcast have been received and correctly used to update the remote replica database.
Another system for updating information in a database is described in U.S. Pat. No. 5,491,820 to Belove, et al. (the "'820 Patent"). The '820 Patent is directed to a client/server system that uses an object-oriented database at the server to provide data to intermittently coupled clients. In this system, data objects are maintained in the database. These data objects may be transmitted to a client coupled to the server and later updated upon re-establishment of communication with the server. Upon receipt of a data object, a client associates a time of receipt with the data object. In requesting an object update, the client transmits a list of data objects to be updated along with the time of receipt for each object. The server maintains a last modified time for each data object and compares the time of receipt for a data object in a data object list received from a client to its last modified time. The server provides new versions of these data objects that have been modified since the last time of receipt.
In a typical synchronization system an individual file containing changes to be sent is created on the server for each client within the system. This ties up the server, increasing the amount of processing done on the server to create these files which is directly proportionally to the number of clients in the system. What is needed is a database synchronization system that allows the database to be scaled for use by increasing the numbers of clients without negatively impacting server performance.